Combined psychophysical and neurophysiological studies of tactual stereognosis will be carried out in monkeys and humans in order to investigate the role of the primary somatosensory cortex (SI) in the tactual exploration and identification of the shapes of three-dimensional objects applied to the fingerpad. Humans will discriminate between and/or identify the orientations of shapes consisting of ellipsoid, cylindrical bars or spheres under stimulus conditions that differ in the trajectory characteristics and the degree of control the subject has over the object's movements. In contour following tasks, monkeys and humans will control the position of the object by the isometric forces generated by the restrained fingerpad. Evoked responses in SI cortical neurons in the monkey will be recorded during the contour tracking task or in response to the different shapes stroked across the monkey's passive fingerpad under a variety of stimulus conditions. Response selectivity will be determined for different shapes and their orientations and to different contact forces, stroke orientations and stroke velocities. In other experiments, humans will judge the misalignments or disparities between the loci of shapes applied to the thumb and index finger, held in a precision grip, while separate shapes are presented to each finger. Recordings from cells in SI in monkeys will be made as the same shape stimuli are stroked across the distal pads of the thumb and index finger passively held in a precision grip. We will study the cortical mechanisms in SI contributing to the fusion of tactile and kinesthetic information from two fingerpads into a unified neural representation of shape.